1. Field of the Invention
This invention relates to switched reluctance motors and, more specifically, to a switched reluctance motor that has an indirect rotor position sensor and a magnetic brake feature.
2. Description of the Prior Art
Switched reluctance motors are used in a variety of applications. One such application includes an automobile hydraulic control system. Such systems are typically controlled by, for example, a piston that is driven by a motor through a rotary-to-linear converter. One example of such a mechanism is a ball nut and lead screw mechanism. Switched reluctance motors are useful in such applications because they can operate over a wide speed range. Switched reluctance motors are also useful because they operate from a low voltage source, typically found in automobile systems. Further, switch reluctance motors have a high torque-to-inertia ratio compared to conventional dc motors, which provides for fast system response time.
The use of conventional switched reluctance motors in such applications has several limitations and includes several shortcomings and drawbacks. In order to operate at high speed from a low voltage source, a switched reluctance motor necessarily include conductive coil windings with relatively few turns of wire about each stator pole. The inclusion of a low number of turns results in large steady state holding currents under conditions where the motor must maintain a high system pressure. In order to accommodate such high holding currents, the motor drive electronics must include a large heat sinking capacity. This introduces undesirable additional hardware and expense. For example, a forced liquid or air cooling system must be included in order to compensate for the large steady state holding current that is required.
Another disadvantage associated with conventional switched reluctance motors is that a separate mechanical brake or clutch mechanism is utilized to lock the motor shaft in a particular position when the motor must maintain a high system pressure. Although the inclusion of such a mechanical brake mechanism reduces the electronic thermal dissipation requirements, it introduces additional hardware into the system, which reduces ease of assembly, conservation of space and other efficiencies.
Switched reluctance motors are electronically commutated, therefore, some means of rotor position sensing must be provided. Prior attempts at determining rotor position include direct position sensing devices such as optical encoders or resolvers that measure rotor position. Such devices are often undesirable because they introduce additional hardware into the system and, therefore, additional cost. Several attempts at using an indirect rotor position sensing methodology include using a non-torque producing main phase winding to generate a position signal. A major disadvantage associated with such prior indirect sensing methods includes the necessary use of a relatively small test current to generate a rotor position signal. Since the non-torque producing stator pole is not electrically isolated from the remainder of the system, there is an inherent possibility for interference from electrical noise. Since the test current used is necessarily small, the noise-to-signal ratio can be undesirably small. In other words, such indirect sensing methods include an undesirably high possibility for the introduction of error into the rotor position determination.
Therefore, this invention provides a switched reluctance motor and methodology that overcome the drawbacks discussed above. A switched reluctance motor designed in accordance with this invention and utilized in connection with the inventive methodology provides many significant advantages.